Among flat display devices, a liquid crystal display (LCD) or an organic light emitting display (OLED) is an active device including a thin film transistor formed on a surface of a glass substrate. In general, such a thin film transistor is fabricated through various steps of depositing an amorphous silicon thin film on a surface of a transparent glass substrate or a quartz substrate, crystallizing the amorphous silicon thin film into a crystalline silicon thin film, and activating the crystalline silicon thin film by implanting a dopant into the crystalline silicon thin film.
Generally, the amorphous silicon thin film is deposited on the glass substrate through a chemical vapor deposition (CVD) process and is crystallized into a poly-crystalline silicon thin film through a predetermined heat treatment process. In addition, the dopant is doped into the poly-crystalline silicon thin film in order to activate the poly-crystalline silicon thin film.
The amorphous silicon thin film deposited on the glass substrate can be crystallized through various crystallization processes, such as a solid phase crystallization (SPC) process, a metal induced crystallization (MIC) process, and an excimer laser crystallization (ELC) process.
According to the SPC process, a heat treatment process is carried out at a predetermined temperature in order to crystallize the amorphous silicon thin film. In general, the glass substrate formed with the amorphous silicon thin film is subject to the heat treatment process at the temperature above 600° C.
According to the MIC process, a predetermined metal element is added to the amorphous silicon thin film in order to crystallize the amorphous silicon thin film at a relatively low temperature. However, if the heat treatment temperature is too low, a grain size may be reduced and crystallinity thereof may be degraded, thereby lowering driving characteristics of the semiconductor device. In particular, if the added metal element is introduced into a channel area of a transistor, current leakage may be increased. In order to solve the above problem of the MIC process, a metal induced lateral crystallization (MILC) process has been developed. However, according to the MILC process, the heat treatment process must be carried out at the temperature above 500° C. in order to induce the lateral crystallization.
According to the ELC process, the amorphous silicon thin film formed on the glass substrate is instantly melted by means of high-energy laser irradiated onto the amorphous silicon thin film and the melted silicon thin film is again cooled, thereby crystallizing the amorphous silicon thin film. Although the ELC process can crystallize the amorphous silicon thin film without deforming the glass substrate, a line pattern bonding may be created due to the high-energy laser or an uneven crystalline phase may occur if high-energy laser is unevenly irradiated onto the amorphous silicon thin film, thereby lowering driving characteristics of the semiconductor device. In addition, the ELC process requires expensive equipment, so the ELC process increases the initial investment cost while presenting limitations to produce the semiconductor devices in mass production.
In the meantime, after the above crystallization process has been performed, the thin film transistor employing the poly-crystalline silicon thin film may be subject to additional processes of implanting a predetermined metal element into the poly-crystalline silicon thin film as a dopant and activating the dopant.
In general, a dopant, such as arsenic, phosphorus, or boron, is doped into a predetermined portion of the poly-crystalline silicon thin film through an ion implantation process or a plasma doping process in order to form an n-type area or a p-type area, such as a source and drain area, in the thin film transistor. After that, the dopant is activated by means of laser or the heat treatment process.
Similarly to the crystallization process for the amorphous silicon thin film, the laser or the heat treatment process is carried out in order to activate the dopant. For instance, an excimer laser annealing (ELA) process, a rapid thermal annealing (RTA) process, or a furnace annealing (FA) process can be performed to activate the dopant.
According to the ELA process, the dopant is activated through a mechanism identical to that of ELC process used for crystallizing the amorphous silicon thin film, in which the dopant may be activated while the amorphous silicon thin film is being rapidly melted and crystallized by means of nano-second pulses. Thus, the ELA process also has the problems represented in the ELC process. That is, the ELA process may cause thermal stress to the poly-crystalline silicon thin film if the amorphous silicon thin film is unevenly crystallized or melted due to high-energy laser unevenly irradiated onto the amorphous silicon thin film, thereby degrading reliability of the semiconductor device.
According to the RTA process, the glass substrate is subject to the heat treatment process at the temperature above 600° C. by using a heating source, such as a tungsten-halogen lamp or an Xe arc lamp. However, the RTA process may cause deformation of the glass substrate if the heat treatment process is continuously performed more than several minutes at the temperature above 600° C. In contrast, if the heat treatment process is performed at the temperature below 600° C., the dopant cannot be sufficiently activated, thereby degrading the characteristics of the semiconductor devices.
According to the FA process, the glass substrate formed with the poly-crystalline silicon thin film is maintained under an atmosphere having a predetermined heat treatment temperature for several hours, thereby activating the dopant. However, the FA process cannot sufficiently activate the dopant if the heat treatment temperature is lowered, thereby degrading reliability of the semiconductor devices. In addition, since the FA process requires several hours of process time, productivity thereof may be lowered.
When the above crystallization processes or the dopant activation processes for the amorphous silicon thin film are carried out, the heat treatment temperature may exert a serious influence upon process time, quality of the poly-crystalline silicon thin film, and reliability of the semiconductor devices.
Generally, the glass substrate used for the LCD or the OLED is a boro-silicate based glass substrate, in which fluidity of glass may increase if it is exposed to an atmosphere having a temperature above 500° C. for a long period of time, so that mechanical strength of the glass substrate is degraded while causing thermal deformation thereof. If a local temperature deviation occurs in the glass substrate, the glass substrate may be seriously deformed or damaged. That is, an inner portion, an edge portion, and an outer portion of the glass substrate may be heated or cooled with mutually different heating or cooling speeds, so a temperature difference may occur therebetween. Thus, the glass substrate is subject to thermal stress due to the temperature difference, so that the glass substrate is easily deformed. In addition, even when the glass substrate is maintained in a constant temperature, if the glass substrate has an uneven temperature distribution, the glass substrate is deformed due to thermal stress or the glass substrate is unevenly shrunk due to the densification behavior, causing deformation of the glass substrate.
For this reason, it is necessary to provide a device capable of preventing deformation of the glass substrate caused by a local temperature deviation and uneven thermal stress when the glass substrate is heat-treated at the temperature above 600° C.
Conventionally, a horizontal continuous furnace or a vertical tube furnace has been used for heat-treating the glass substrate through the SPC process or the MIC process. The glass substrate is moved along an interior of the horizontal continuous furnace having a length of about several tens of meters by means of a conveyer or a roller for the heat treatment process. The horizontal continuous furnace smoothly increases or decreases the temperature of the glass substrate in order to prevent the glass substrate from being deformed or damaged during the heat treatment process, so the total length of the horizontal continuous furnace may be increased. For this reason, the horizontal continuous furnace requires long process time of about several hours or several tens of hours for the heat treatment process. Due to the long process time for the heat treatment process, the horizontal continuous furnace may not significantly increase the heat treatment temperature to avoid the deformation of the glass substrate.
In the meantime, the vertical tube furnace simultaneously performs the heat treatment process with respect to a plurality of glass substrates by vertically mounting plural glass substrates on a quartz boat or a silicon carbide (SiC) boat installed in the vertical tube furnace. The glass substrates are heat-treated by means of heat applied to outer portions of the glass substrates. Thus, the temperature difference may occur between outer portions and inner portions of the glass substrates. In particular, in cases of large-sized glass substrates, there is a great difference of heating and cooling speeds between the outer portions and the inner portions of the glass substrates, so that the glass substrates may be seriously deformed. In addition, contact portions of the glass substrates, which make contact with the boat, may represent the heating and cooling speeds different from those of non-contact portions of the glass substrates, which do not make contact with the boat, so that the glass substrate is unevenly heated or cooled. Therefore, the vertical tube furnace must slowly heat or cool the glass substrates (for example, it heats or cools five glass substrates per one minute) in order to reduce the difference of the heating and cooling speeds between the inner and outer portions of the glass substrates, so relatively long process time is required. In addition, since the glass substrates are installed in the vertical tube furnace while being supported by the boat, if the heat treatment process is performed for a long period of time at the temperature above 500° C., the glass substrates may droop due to the tare thereof, so the vertical tube furnace is not applicable for the dopant activation process or the SPC process, which are performed at the temperature above 600° C., but only applicable for the heat treatment process performed at the temperature below 500° C.